Evanescent near-field optical lithography : overcoming the diffraction limit.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Concepts of optical resolution limits have been transformed in the past two decades with the development of near-field optical microscopy. Resolutions of λ/40 have been demonstrated by taking advantage of additional information present the near field of an object. These resolutions are far higher than what diffraction-limited lens-based optical systems are capable of. Attempts have been made to replicate these resolutions for lithography using a scanning probe based optical equivalent, but these systems suffer from low throughput owing to their serial nature. A desirable alternative would be replication of all the patterns within a field in a single flood exposure in a manner similar to how optical projection lithography replicates the field of a mask, but with the additional resolution available from working in the near field. This is the basis of evanescent near-field optical lithography, the subject of this thesis. Evanescent near-field optical lithography (ENFOL) brings traditional contact lithography into the near-near field using a combination of conformable masks and ultra-thin photoresists. This thesis describes a study of ENFOL both experimentally and via electromagnetic simulations to evaluate what the resolution limit might be. The fabrication of membrane masks is described, a key component for the ENFOL exposure. The characteristics of an ENFOL exposure using broad-band light are investigated from exposures into thick resist. These exposures demonstrate the trend of decreasing depth of field as the period of grating structures is reduced. ENFOL's requirement of a thin imaging photoresist for high resolution lithography complicates the pattern transfer step essential to translate the photoresist image into a useful material for devices. The development of an additive pattern transfer process is described, that utilises a trilayer resist scheme to enable lift-off metallisation. NiCr gratings with periods down to 270nm have been fabricated using this process subsequent to an ENFOL exposure. Wire-grid polarisers consisting of 270nm-period NiCr gratings on glass substrates have been fabricated and their polarisation properties measured at visible wavelengths. Simulation results of exposures of sub-wavelength grating structures are presented that investigate the fundamental limit to resolution for contact lithography techniques such as ENFOL. A full-vector, rigorous electromagnetic simulation technique, the multiple multipole program is used to provide information about the near field of subwavelength gratings. The potential for λ/20 resolution is indicated; a tantalising prospect for optical lithography and well below the diffraction limit of conventional optical projection-based lithographies. Perhaps the most critical parameter for an evanescent exposure, the depth of field, was characterised and a linear relationship shown between the depth of field and grating period. The effect of parameters such as grating duty cycle, absorber material and thickness on the exposure are observed with the intention to optimise the experimental setup. Interesting interference phenomena are observed in simulation results for exposures. where the effective exposure wavelength is equivalent to the grating period. In particular a period halving occurs in the transverse magnetic polarisation due to interference of the first diffracted orders. A novel interference technique - evanescent interference lithography is proposed that takes advantage of an enhanced period halving at an exposure wavelength corresponding to a grating resonance.